Lithium rechargeable batteries are the premier energy storage device for portable electronics applications. However, a significant need remains for the improvement of the energy density of the cells. The main limiting factor to the realization of such energy density increase is the positive electrode materials.
All lithium secondary batteries utilize positive electrodes of intercalation compounds which retain their crystal structure upon lithium insertion. The host structure remains intact even though the lattice may expand, contract or distort slightly upon Li′ insertion. The transition metals present in all of the intercalation compounds are capable of multiple electron transfer and thus higher capacity, however, the limited lithium vacancies inhibit the incorporation of more lithium and thus the charge transfer of more electrons to the structure. Further, the covalency of the transition metal dichalcogenide bond reduces the voltage of the reaction. The most popular transition metal dichalcogenides have a common crystal structure and form a group of layered, highly anisotropic compounds.
In contrast to the intercalation process, the conversion process enables full utilization of all the redox potentials of the host metal as it reduces fully to the metallic state. In the specific case of metal fluorides, this transition behaves quasi-ion-like with redox potentials approaching that of free ions in solution due to the fact that metal fluoride compounds are highly ionic. The metal fluoride conversion reaction leads to LiF and metal (Me) products which are on the scale of 2-5 nm. Reversibility and thus reformation of the MeFx structure can occur on the following charge due to the extremely small diffusion distances between these thermodynamically very stable reaction products. The practical result is the theoretical improvement of the specific capacity of the positive electrode from 274 mAh/g for layered intercalation compounds to >700 mAh/g for the reversible conversion metal trifluorides. This may be represented according to formula [1]:xLi++xe+MeFx←→xLiF+Me  [1]
The physical proof of the ability of fluorides to reversibly convert has been demonstrated by the separate but parallel efforts of Hong (Li, H., et al. Adv. Mater. 15:736-739. 2003) and Badway (Badway, F., et al. J Electrochem. Soc. 150(10):A1318-A1327. 2003; Badway, F., et al. J Electrochem. Soc. 150(9):A1209-A1218. 2003). Hong's work involved the use of a TiF3 compound. Reversible conversion was confirmed through the use of Raman spectroscopy which suggested the reformation of TiF3 while the electrochemical data showed multiple cycle reversibility. Badway's approach focused on higher voltage metal fluorides which typically are more insulating. FeF3, FeF2, NiF2 and CoF2 nanocomposites were fabricated with conductive carbon matrices in order to enable their electrochemical properties. These nanocomposites were of relatively low surface area with nanocrystalline regions of MeFx (10-30 nm) encapsulated by an amorphous carbon matrix. Specific capacities in excess of 600 mAh/g and 400 mAh/g were demonstrated at 70° C. for FeF3 and NiF2, respectively. This approach has lead to the realization of the theoretical voltage of a wide variety of compounds including FeF3, FeF2, NiF2, CoF2, as well as CrF3, CrF2 and BiF3.
The utilization of nanocomposites realizes near theoretical specific capacity with theoretical volumetric energy densities approaching that of CFx. The exceptional rate capability of these materials and their reversibility has been shown to occur through the use of in-situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Reversibility was further shown by prefabricating 3LiF+Fe nanocomposites and cycling, thereby liberating lithium during the first charge to be utilized in a Li-ion configuration.
Many of the MeF3 structures are related to PdF3—ReO3 structures based on a primitive cubic cell unit that consists of corner shared MeF6 octahedrons and empty “A” sites. There is little to prevent shearing of these crystal structures hence there is a large range of crystallographic distortions upon moving from the ReO3 structure to the PdF3 structure. These structures form vacant octahedral interstices that allow the diffusion of lithium ions. Cation vacancies allow many MeF3 materials to support intercalation and thereby form LixMeF3 compounds such as TiF3, FeF3, and VF3 (Arai, H., et al., J Power Sources. 68:716. 1997). It has been demonstrated in other works that the MeF3 compounds supported a 1e− intercalation region followed by the 2e− conversion reaction resulting in the following reaction [2]:Li++e−+MeF3←→LiMeF3 2Li++2e−+LiMeF3←→3LiF+Me  [2]
In the case of FeF3 nanocomposites, the intercalation mechanism was found to be quite fast and very reversible.
Studies have reported the synthesis of metal fluoride nanocomposite electrode materials utilizing a novel mechanochemical induced reaction. High energy milling of the insulating CF1 and MeF2 compounds resulted in a solid state redox reaction with the oxidation of the MeF2 compound into MeF3. This reaction was induced by the oxidation of the MeF2 compound to MeF3 by the oxidizing power of HT (high temperature fabricated) CF1. The resulting product was a fine nanocomposite of MeF3 in a matrix of conducting carbon that may be represented according to the following reaction [3]:CF1+MeF2→C+MeF3  [3]
The mechanochemical induced oxidation reaction was successfully carried out for the reaction CrF2→CrF3 and FeF2→FeF3. Such materials exhibited good reversibility and excellent capacities in excess of 600 mAh/g and 500 mAh/g, respectively. This technique worked for all metal fluorides with Me2+→Me3+ redox levels below that of the theoretical oxidizing power of CF1.
Previous methodologies for the fabrication of iron fluoride nanocomposites with the highest specific energy density consisted of the high energy milling of various iron fluoride components with a conductive matrix, such as carbon. Although such nanocomposites have imparted marked improvements in energy density relative to existing materials, significant improvement in the rate capability (power density) of the material is needed. Additionally, improvement in the cycling efficiency of the material such that the material can be discharged and charged repeatedly with very little cycle to cycle capacity fade also is needed.
One novel approach is the use of oxyfluoride materials of iron as the active electrode materials. Although such materials have been isolated before, none have been examined as possible electrode materials for lithium batteries. Although Brink et al. (F. J. Brink, R. L. Whiters and J. G. Thompson, J. Solid State Chem., 155, 359-365. 2000) previously reported the solid state synthesis of FeOxF2-x solid-solutions utilizing FeF2 and FeOF precursors, a significant disadvantage of this fabrication technique is that it requires the preliminary synthesis of FeOF, which cannot be formed below 925° C. Moreover, synthesis of the FeOxF2-x solid-solutions was performed under controlled atmosphere and very high temperatures (850° C. for 3 hours) that resulted in the formation of macrocrystalline compounds.
The present invention provides herein a new positive electrode material for electrochemical energy storage and a solution fabrication process for the synthesis of nanostructured iron (oxy)fluoride materials from iron metal and fluorosilicic acid (H2SiF6) aqueous solutions. The solution synthesis rationale comprises the fabrication at low temperature of nanostructured iron (oxy)fluoride materials FeOxF2-y, with compositions ranging over the entire range from FeF2 to FeOF, utilizing inexpensive, commercially available precursors. The formation of a FeSiF6 hydrate intermediate and its subsequent anneal in air are steps critical for the formation of the (oxy)fluoride materials.